The present invention generally relates to a resonant magnetic deflection circuit, more particularly, to a horizontal magnetic deflection circuit for a raster scan type cathode ray tube display apparatus.
A raster scan type cathode ray tube (CRT) display is widely used as a picture monitor, or as a display apparatus in a computer display terminal, or the like. For displaying an image on the CRT, an electron beam generated therein is scanned across the CRT, and, simultaneously therewith, is modulated and deflected in a horizontal and a vertical direction. There are two types of deflection circuits. One of the circuits is an electrostatic deflection circuit, the other being an electromagnetic deflection circuit. The latter is popular for the raster scan type display apparatus in comparison with the former because of its high deflection efficiency. In the magnetic deflection circuit, a ramp current flows through a yoke, namely, a deflection coil. A resonant magnetic deflection circuit is generally used as a horizontal deflection circuit. A conventional resonant magnetic deflection circuit is disclosed on pages 20-52 and 20-53 of "Electronics Engineer's Handbook, First Edition" edited by Donald G. Fink and published by McGraw-Hill Book Company in 1975. This conventional resonant magnetic deflection circuit comprises a switching transistor controlled in accordance with a horizontal drive signal, a damper diode and a retrace capacitor connected in parallel with the switching transistor, a series circuit consisting of a deflection coil, an S-correction large capacitor connected in series with the switching transistor, and a voltage source connected to a common junction of these devices through an inductor, such as a flyback transformer. The retrace capacitor and the deflection coil form a resonant circuit, so that energy from a power supply is converted to an electromagnetic energy of the deflection coil for the latter half of a scanning (trace) period, the electromagnetic energy being converted to electrostatic energy of the retrace capacitor for the first half of a retrace period, the electrostatic energy being converted to the electromagnetic energy again for the latter half of the retrace period, the electromagnetic energy returning to the power supply for the first half of the scanning period. Thus, efficiency is very high. Moreover, a deflection coil and an S-correction capacitor comprises a resonant circuit for correcting S-distortion. Therefore, the resonant magnetic deflection circuit is widely used.
On the other hand, the inductance (Ly) of the deflection coil is determined by the power voltage (Vc), the scanning period (ts) and the deflection efficiency (E), i.e., Ly=(Vc.multidot.ts).sup.2 /E. The retrace period tR is defined to be equal to .pi..sqroot.LyC.sub.R, wherein C.sub.R is the capacitance connected in parallel with the series circuit including the deflection coil. It is desirable to reduce the retrace period, because the scanning period can be increased as a result thereof, since the sum of the scanning and retrace periods is determined to be a horizontal period (for example, 63.5 .mu.s) of a video signal standard. Thus, the retrace period directly affects video bandwidth and CRT brightness requirements. If these requirements are reduced, it may be easy to manufacture a very high resolution video display apparatus.
As described hereinbefore, the retrace period is determined by the variables Ly and C.sub.R, the variable L.sub.y being determined by many factors. The retrace period can be reduced by decreasing the capacitance C.sub.R.
When Ly energy is transferred completely to C.sub.R, then 1/2LyI.sup.2 =1/2C.sub.R V.sup.2, I=peak deflection current (determined by other factors), V=peak voltage across C.sub.R. Since t.sub.R =.pi..sqroot.LyC.sub.R, it follows that ##EQU1##
The peak voltage across C.sub.R then increases in proportion to 1/t.sub.R. Therefore, the retrace capacitor, deflection yoke, switching device, damper diode, flyback transformer, and all other circuit elements connected to the retrace capacitor must withstand the higher peak voltage if retrace period is to be reduced by decreasing C.sub.R.
However, the flyback transformer has a large stray capacitance, and the capacitance of variable C.sub.R is determined by a sum of the stray capacitance and the retrace capacitor's capacitance. The capacitance variable C.sub.R cannot be less than the stray capacitance. If the retrace capacitor is removed from the deflection circuit, the capacitance C.sub.R may be unstable. Thus, it is difficult to reduce the retrace period by decreasing the capacitance C.sub.R.
In a higher resolution raster type display and in a stroke-writing type CRT display, a deflection yoke contains two physically separate, series connected, horizontal windings (deflection coils) with a core and vertical windings adjacent to both the horizontal windings. A horizontal retrace pulse of the resonant deflection circuit can produce a damped oscillation in the vertical windings. This phenomenon is called "yoke ringing". This "yoke ringing" can appear on a display screen as a set of wavey horizontal lines.